I agree with Mr. Naegeli as far as the unlikelihood of a "knock-off" DB ever happening. There were some Alfa vehicles that were copied, but as far as I know never a 750 GT.
It has been common practice on Ebay for listers to include the word "Abarth" in an attempt to coax some extra returns from the sale. It is up to the bidders to do their homework and decide what is real and what is not. Personally, any car that is listed on Scuderia Topolino is marked as a real Abarth, if it is so. If not, then it is described as a replica or re-creation.
Unfortunately, a great many "real" Abarth cars, of the 850TC and 1000TC variety, were updated to later specifications by the factory in order to keep them competitive. This makes the process of authentication very difficult. I am sure that all of the major collections of Abarth cars (Moell, Pont, Aumuller etc) vet the purchases that they make very thoroughly, particularly on the sedan based Abarth cars.